NATO Secretary-General Anders Fogh Rasmussen has warned Russia that any incursion into eastern Ukraine would be a "historic mistake," and demanded that Moscow pull back its forces from the Ukrainian border.

Speaking April 8 in Paris, Rasmussen said further intervention by Russia in Ukraine would have "grave consequences" for Moscow's relationship with NATO and "would further isolate Russia internationally."

Rasmussen called on Russia pull back what he described as “tens of thousands” of troops massed near the Ukrainian border.

His comments came as Ukrainian authorities were moving to counter pro-Russian protesters who seized government buildings in three cities in eastern Ukraine.

Authorities in Kyiv said they seized back control of the regional administration building in the eastern city of Kharkiv from pro-Moscow separatists, and arrested about 70 "separatists."

Authorities also said they hoped to soon regain control of buildings in Luhansk and Donetsk.

In Luhansk, pro-Moscow protesters occupying the state security building have blocked neighboring streets with barbed wire and piles of tires.

Standoff In Donetsk

A standoff was also continuing in Donetsk, where separatists occupying the regional government building on April 7 declared a "sovereign people's republic" and called on Moscow to send in Russian "peacekeepers."

Acting Ukrainian President Oleksandr Turchynov on April 8 blamed Russia for the unrest in the east and vowed that authorities will prosecute “separatists” who break the law as “terrorists and criminals.”

Ukraine's parliament on April 8 passed a bill to increase the punishment for separatist activities.

In Moscow, Russia's Foreign Ministry warned in a statement that any use of force against pro-Russian protesters in the east could drag Ukraine into civil war.

Meanwhile, Foreign Minister Sergei Lavrov has rejected U.S. accusations that Russia has been working to destabilize eastern Ukraine.

Speaking in Moscow on April 8, Lavrov also said Russia is ready to participate in proposed talks involving the Ukrainian authorities, the United States, and the European Union.

Lavrov said the situation in Ukraine will improve only if the concerns of Ukraine's Russian-speakers are addressed.

Lavrov reiterated Moscow's call for constitutional reforms in Ukraine to guarantee the rights of Russian-speakers.

Lavrov’s comments came a day after U.S. Secretary of State John Kerry said Russia will face “further costs” if it is responsible for destabilizing Ukraine.